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Making a Hard-Shell Mold 


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Making a Hard-Shell Mold 

Written By: Adam Savage 


• Clay-sculpting tools (1) 
wire loop, carver, etc. 

Hot glue gun (1) 

Marker (1) 

Paintbrush (a few) 

Pencil (1) 

Putty knife (a few) 
or screwdrivers 

Ra qs(1) 

Rolling pin (1) 

Squirt bottled) 

X-Acto knifed) 


• Model piece (1) 

the original that you want to cast 

• Aluminized tape (1) 

• Silicone rubber (1) 
for moldmaking 

• Silicone rubber thickener (1) 

• Silicone rubber accelerator (1) 

• Silicone sealant (1) 

• Spun hemp (1) 

• Stone plaster (1) 

• Casting resin (1) 

• Water-based pottery clay (1) 

• Foam core (1) 

• Plastic hemispheres (1) 

available at any plastics supply store 

• Vaseline (1) 

or mold release 

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Making a Hard-Shell Mold 

Mineral spirits (1) 


Silicone block molds are fine for making casts of small objects. But for larger items, like this 
3' prop gun, you'd need hundreds of dollars' worth of silicone to make a block mold. 

An excellent and inexpensive solution is to use a thin layer (or "blanket") of silicone, that's 
keyed to a hard-shell or "mother" mold. It's a multi-step process, but it yields great results 
for the cost-conscious mold maker. It also makes much lighter molds, which are easier to 
move around. 

With this type of mold making, you're basically sculpting the 2 sides of your mold, taking into 
account the forces involved in the pouring and casting of the part. It takes a while, but if you 
get good at hard-shell mold making, you can cast just about anything, no matter how big. 

The silicone is poured into 2 
halves, and each half registers into 
its respective mother mold. Before 
doing anything else, take a 
permanent marker and draw a 
parting line down the exact middle 
of the original part, marking 2 
symmetrical halves. This is the line 
you'll sculpt everything to — even 
with simple objects like this, every 
mold maker I know does it, and you 
should too. 

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Making a Hard-Shell Mold 

• Before we make the mold itself, we 
need to make your original model 
mold-worthy. To prep it, you'll need 
to pre-fill up any small or difficult- 
to-access voids in the model, such 
as the hole in the bottom of our 
prop gun body, which you can see 
in this image (circled). This is a 
void on the model that has threads 
to join to another part of the model. 

• I knew that if I poured that void in 2 
parts and from the side, I'd end up 
with air bubbles galore. So I pre- 
filled it with a plug made of blue 
silicone. The plug should stick to 
the blanket of silicone that I pour 
later, and become a nice part of the 

Prior to pouring the silicone, A^fr 
you'll need to clean the 1*1 

plugs thoroughly with mineral 
spirits to make sure they'll stick. 

© Make Projects 

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Making a Hard-Shell Mold 

• Next, we'll make a table out of 
foam-core on which to sculpt the 
first half of the mold. Use a piece 
of foam-core that's large enough to 
extend at least 8"-10" beyond the 
borders of the part. (A common 
mistake people make with these 
molds is not giving themselves 
enough surface area to work on.) 

• Place the model on its side with the 
parting line parallel with the foam- 
core and trace around it with a 
pencil, as close to the model as 

• Cut the model shape out of the 
foam-core and discard. 

• Secure the model on a sturdy work 
table (oil-based clay is great for 
this step), making sure the parting 

ine is as level as possible. 

Now, we need to position the foam- 
core so that it sits about Vi" below 
the centerline of the model. We'll 
be creating a clay dam to use as a 
reservoir for the silicone, and that 
VV allows for the thickness of the 
clay to come right to the middle 
parting line. 

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Making a Hard-Shell Mold 

• Measure from the parting line to the 
table. Let's say it's 2". 

• Subtract W for your clay dam, 
which gives us 1%". 

• Subtract the thickness of the foam- 
core (3/16"), which takes us to 1- 

• Cut a few dozen little strips of 
foam-core 3"^" long to this exact 
height, in our case 1-9/16". 

• Place the model on the table, and 
spread the upright strips around it 
evenly, covering an area the size 
of the cut-out foam-core piece. 

• Use hot glue to adhere the upright 
chunks to the table and to the large 
foam-core piece, which should be 
placed on top of them to create a 
nice solid foam-core surface. 

© Make Projects 

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Making a Hard-Shell Mold 

• Now you need to decide where the 
top of your mold will be, into which 
you'll pour the casting resin. It 
should be located at the perimeter 
of the model where it comes to a 
steep point, so bubbles will surface 
and pop in a small area. 

• Because you'll be pouring into this 
part, it's a good idea to add a piece 
to your foam-core table that gives 
you a nice clean surface for 
pouring the resin into the mold. And 
you'll need a vent nearby to give 
the air you're displacing with the 
casting resin somewhere to go 
(other than back through the hole 
you're pouring from). This helps 
eliminate bubbles, and makes 
pouring the mold a cleaner, less 
splash-prone process. 

• In this image, you can see the 
funnel-shaped pouring gate and 
next to it a smaller vent. 

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Making a Hard-Shell Mold 

• Since you want the clay to be a uniform thickness, a rolling pin is ideal for the job. I 
use water-based clay, the kind used for pottery, because oil-based clay and 
silicone don't always get along. 

• Flatten the clay with your hands, and when it's a little more than W thick, place the clay 
pieces between 2 rails (wood will do) that are exactly W high, positioned perpendicular to 
the rolling pin and close enough together that they're underneath its rolling surface. 

• Roll the clay to a Vi" thickness — use a squirt bottle of water to keep it from getting sticky, 

• The first image shows the clay dam, mostly laid out onto the foam board. It goes on in 
pieces, which you can join together with your fingers. 

• Use a clay tool to bring the clay dam right up to the model. 

• The line where the clay meets the model should be very smooth, perpendicular, and 
have no gaps. The cleaner this area is, the easier it will be to get good castings. 

• After finessing the clay dam, clean all clay residue off the model with a damp brush 
(otherwise it will be cast in when you pour your silicone!). 

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Making a Hard-Shell Mold 

• Use a wire-loop clay tool to make 
the first of the "barrier keys" (or 
more accurately, registration 
topography) that will hold the 2 
halves of this mold perfectly 

• Keys for molds come in 
many shapes and sizes, but 
for large molds, a key that runs all 
the way around the part helps 
prevent the resin from leaking out. I 
usually go a bit deeper than a half- 
circle's depth, then clean the edges 
with a wet soft paintbrush. 

• This image shows the completed 
barrier key around the part, and a 
raised clay border (made using the 
rolling pin method described in the 
previous step) built up all the way 
around the part, with spacing of 
about 3". 

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Making a Hard-Shell Mold 

The first layer of silicone is the most important one, because it's what grabs all the detail 
from your model. To avoid bubbles, pour slowly from one location and from high up, letting 
the silicone drift slowly into the detail on the model. 

You can use accelerator in the silicone (or use more kicker) to make it kick faster, 

but that will make for a weaker mold. If you need only 1 or 2 castings, it's OK to 

use an accelerator (they can speed up the setting time from 10 hours to 3), but if you want 

to make dozens of castings, be patient. 

With the first thin layer covering the model, blow compressed air over the part (don't get 
too close) to eliminate any bubbles. 

When the first layer is just past the tacky stage, brush on another layer of silicone, making 
sure it's of uniform thickness all over the model. 

For this application, you can add thixotropic agents to increase the silicone's 
viscosity (but not on the first layer, as these agents make it difficult to get the 
silicone into all the nooks and crannies for high detail). 

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Making a Hard-Shell Mold 

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Making a Hard-Shell Mold 

Once the second layer is applied 
for the first blanket of silicone, and 
while that layer is still wet and 
tacky, start applying the silicone 
keys. These will help the silicone 
stay adhered to the inside of the 
hard plaster mold. While the 
advantage of this method is that it 
uses less silicone, the 
disadvantage is that the thin 
silicone layer lacks structure and 
must be married to the plaster mold 
so it doesn't collapse. 

I made these keys from an old 
silicone mold by cutting small 
wedges about IV2" long by about a 
pinky width (make sure that the 
silicone for the keys and the mold 
are the same brand; it helps them 
stick better). For this mold, I set 
keys in the wet silicone about 
every 3". 

As every mold is different, 
you have to imagine your 
mold upright and think through the 
weak spots, where it will buckle, 
and place the keys accordingly. 

This image shows the mold with 
the keys in place all the silicone is 
poured (for the first half), and it's 
setting up nicely. The silicone 
doesn't go all the way to the border 
on the left, but that's OK. 

Next we'll be cutting dovetail keys 
around the perimeter. 

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Making a Hard-Shell Mold 

Step 10 — Cut the dovetail keys 

• After the silicone has cured, trim the edge of the blanket to get a smooth line, and discard 
the trimmed pieces. Use a sharp X-Acto knife to cut dovetail-shaped keys around the 
perimeter, gingerly lifting up the edge of the blanket and slicing upward so you don't cut 
into the clay below. 

• This serrated edge will help the silicone blanket register to the mother mold. My 
blanket here is a wee bit thin at the outer edge. I could probably have trimmed it 
closer, like about an inch away from the model. 

• The second image here shows the finished blanket. I've probably used the minimum 
number of dovetail keys necessary to keep the blanket stable in the mother mold, but you 
should err on the side of caution and add more than you think the mold might need. 

• Too many keys just makes the mold more stable, but too few and you've wasted a 
lot of work. 

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Making a Hard-Shell Mold 

Step 11 — Add hemispherical keys 

• I've left space around the edges of 
the silicone so I can place 
hemispherical keys (pictured here). 
These will register the 2 halves of 
the mother mold together. I'm using 
injection-molded W plastic 
hemispheres, available at any 
plastics supply store, placed lightly 
on the clay every 5" or so, just 
inside the border. 

• Brush a small amount of Vaseline 
or other mold release onto them to 
help remove them from the first 
half of the plaster mother mold. 

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Making a Hard-Shell Mold 

Step 12 — Plaster over the silicone. 

After the first blanket of silicone comes the stone-plaster mother mold. The plaster will go 
on in a couple of layers. The first layer is a thin coat for detail — apply it slowly to avoid 
creating bubbles. The second is supported by a hemp strengthener. 

The first image shows the first layer of plaster applied over the blanket, the clay, and the 
keys. It's fairly thick and will take somewhere around an hour to set. Stone plaster is much 
stronger than regular plaster. You can get away with using less, which keeps your mold 
ightweight, but it's still brittle like regular plaster. 

So the next step is to reinforce it with some spun hemp, available from mold-making 
supply stores. The hemp works much like fiberglass, supplying a matrix that increases the 

plaster's flexibility and makes it shatter-resistant. Add a layer of hemp, then apply the 
second and final layer of plaster. 

Don't wait more than a day between plaster coats, or else the second layer might 
not stick well to the first. Also note that the first layer of set-up plaster will suck 
water from the new layer, making it set faster than the first. 

Once the second layer's set, turn the whole thing over and gently pull off the clay dam, 
keeping the model inside the mold. Take a moment to study what you've done. Isn't it 
pretty? The various mold keys are all visible now: the hemispherical keys in the outer ring 
of plaster, the dovetail keys where the silicone meets the plaster, and the barrier key 
around the model itself. 

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Making a Hard-Shell Mold 

Step 13 — Make the second silicone blanket, 

Before applying a layer of silicone to the other side of the model, you'll have to clean it and 
prepare it well — but be careful not to mess with it too much, as you want as tight a 
registration as possible. Use a brush and soft damp cloth to remove any clay residue. 

Apply mold release (or a thin layer of Vaseline) to the silicone so that the next layer won't 
stick to it. Make sure you cover it all, or else you'll ruin your mold. Silicone loves to stick 
to itself. Apply 2 layers of silicone exactly as you did in Step 6. As before, use very little 
accelerator in the first layer, but you can use more, or a higher mix of the kicker, in the 
second. Remember: the goal is to get a Vi" blanket all around the part. 

As with the first blanket, place the silicone key wedges along the center of the model 
before the second layer of silicone hardens. Cut the dovetail keys from the second layer, 
gingerly lifting up the edge to avoid cutting into the layer below. 

The second image shows the mold with the second silicone blanket done and cut. Now it's 
time to lay on the plaster for the other half of the mother mold. You're almost there! 

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Making a Hard-Shell Mold 

Step 14 — Make the second half of the hard-shell mold. 

To prep for the second plaster, apply mold release to the first plaster half. Again, a thin 
layer of petroleum jelly works great. Cover the inside of all the hemispherical key 
indentations because, again, if they don't release, all your work is down the toilet. 

Using aluminized tape, available in the plumbing or heating section of any hardware store, 
build a simple mold dam to contain the wet plaster and give it a nice crisp edge that 
matches the clay dam from the first blanket. 

Slowly drip the plaster onto the second blanket of silicone. Again, this first layer is for 
detail, and the fewer bubbles it has, the better it will hold. 

Use your hand to spread the plaster over the blanket, making sure it covers everything, 
especially the silicone wedge keys along the center. 

Be careful around the edge of the blanket, where the dovetail keys are! This edge 
may want to lift up, and you don't want to get any plaster under it between the 2 
layers of silicone. Better to drip the plaster gingerly around the edge. This is an important 
point to remember, and the more familiar you are with this whole process, the less likely 
you are to forget a key step and end up wasting your hard-earned time. 

The third image shows the first layer of plaster. Note how well the aluminized tape dam 
holds it in. Also note how clean the workspace around the mold is. This type of 
moldmaking is very detail-intensive, and attention to cleanliness during the molding 
process will quite simply yield a better product. 

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Making a Hard-Shell Mold 

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Making a Hard-Shell Mold 

• Allow the second layer of stone 
plaster (with hemp below it) to dry 
in all its glory. Once it's dry, all you 
have to do is pull off the tape and 
gently pry the halves apart. Since 
plaster is brittle, care must be 
taken not to over-torque the mold, 
lest it crack. 

• Use 2 screwdrivers or sturdy putty 
knives, one leapfrogged in front of 
the other, to proceed down the 
seam. As you go, listen for the 
telltale sound of the halves letting 
go of each other. Go slowly! You 
don't want a cracked mother mold 
before you've even started casting. 

• Making your way down a full side 
of the mold halves should be 
sufficient for a proper separation. 
After a time, you'll hear a sucking 
sound and see that the 2 halves 
have popped apart. The 2 silicone 
blankets should be somewhat 
stuck together now, but simply 
grabbing one of them and pulling it 
off the other should do the trick (if 
you've properly applied the mold 
release.) Then pull out the model to 
reveal your finished mold. 

• This image shows our completed 
mold. Everything worked perfectly. 
Note the lack of air bubbles in the 
positive hemispherical keys on the 
bottom half. These should register 
the 2 halves of the mother mold 
beautifully. And they did. 

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Making a Hard-Shell Mold 

Step 16 — Use your mold 

• This image shows a completed 
assembly of a resin casting from 
this mold. To reduce air bubbles on 
a large part like this, you can first 
pour some resin in each half and 
let it cure, then assemble the 2 
halves and pour a final resin middle 
to get the completed casting. 

• With proper cycling (letting the 
silicone cool down between 
castings — heat kills molds), this 
mold should easily yield 20 or more 
castings before deteriorating. 
Because the blanket is thin and the 
plaster sucks heat out during the 
resin's curing process, it could 
even yield 50 castings. 

This project first appeared in MAKE Volume 24 , page 109. 

This document was last generated on 201 2-1 1 -03 1 2:31 :54 AM. 

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